Trim fat from meat. Mix mustard and pepper. Rub onto meat. Place meat on a rack in a shallow roasting pan. Insert a meat thermometer. Roast in a 325 degree oven until thermometer registers 140 for medium rare (1 1/2 to 2 hours) or 155 for medium (1 3/4 to 2 1/4 hours). Cover with foil and let stand 15 minutes before carving.

Meanwhile, for sauce, in a medium saucepan, cook mushrooms, green onions and garlic in hot margarine until green onions are tender. Stir in flour and thyme. Gradually stir in beef broth. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Stir in light cream or milk, cook and stir until heated through. Thinly slice meat across the grain. Arrange on a serving platter. Pour some sauce atop meat. Pass remaining sauce. Serve with mashed potatoes.